6-ton NASA satellite set to fall

An artist's conception shows the Upper Atmosphere Research Satellite in orbit.

NASA says a defunct six-ton satellite is due to re-enter the atmosphere — with the potential to rain debris upon Earth.

The Upper Atmosphere Research Satellite, or UARS, is expected to come down in late September or early October, the space agency said today in an advisory. "Although the spacecraft will break into pieces during re-entry, not all of it will burn up in the atmosphere," NASA said.

The agency said it's too early to say exactly when UARS will make its final plunge, or exactly where any debris will come down. Russian news reports suggested that Moscow was "in the zone of risk," but that projection was based merely on the inclination of UARS' orbit.

"The orbital track and re-entry location are going to be more refined as the days pass," NASA spokeswoman Beth Dickey told me today.

UARS was deployed from the shuttle Discovery in 1991 to study Earth's atmosphere and its interactions with the sun. The $750 million mission measured the concentrations and distribution of gases important to ozone depletion, climate change and other atmospheric phenomena. NASA says readings from UARS provided conclusive evidence that chlorine in the atmosphere, originating from human-produced chlorofluorocarbons, is at the root of the polar ozone hole.

The satellite was decommissioned in 2005. "They had put it in a disposal orbit at that point, and that disposal orbit reduced its orbital lifetime by about 20 years," Dickey said. One NASA account suggests that the satellite was at one time projected to come down in the 2009-2010 time frame.

NASA says it plans to post updates about UARS' status weekly until four days before the anticipated re-entry, and then daily until about 24 hours before re-entry. Further updates would come at 12 hours, four hours and two hours before re-entry. The Joint Space Operations Center of the U.S. Strategic Command at California's Vandenberg Air Force Base is monitoring UARS' status around the clock, NASA said.

The satellite's current orbit is 155 by 174 miles (250 by 280 kilometers), with an inclination of 57 degrees. NASA said. That means the satellite would have to descend into the atmosphere somewhere between 57 degrees north latitude and 57 degrees south. NASA estimated that the debris footprint would stretch about 500 miles.

"If there is something you think may be a piece of UARS, do not touch it," NASA said. "Contact a local law-enforcement official for assistance."

NASA's advisory emphasized out that the risk to public safety or property "is extremely small."

"Since the beginning of the Space Age in the late 1950s, there have been no confirmed reports of an injury resulting fromre-entering space objects," it said. "Nor is there a record of significant property damage resulting from a satellite re-entry."

Update for 11:25 a.m. ET Sept. 9: I've updated this item as well as the headline to reflect NASA's latest figure for UARS' mass. Its total dry mass is 5.7 metric tons, or 6.27 English tons, says NASA's Nick Johnson.